We want to find out how the timing of initiation of rounds of DNA replication is controlled in bacteria. Towards that end, a research strategy is proposed to isolate conditional mutants defective in the synthesis of gene products required for initiation of DNA replication. Study of such mutants will enable us first to determine some general features of this control (repressor or specific initiator protein) and in the future to study its mechanism. Initiation and termination of rounds of DNA replication are among the main determinants of the cell cycle (including the cell size) and are thus important for our understanding of cell growth and cell division.